Exhaust gases are emitted by industrial sources such as power plants and incinerators as well as transportation sources such as airplanes and cars. Nitrogen oxides, as a major component of exhaust gases, contribute to the formation of photochemical smog and acid rain when discharged into the atmosphere. Thus, much research has been devoted to developing techniques for efficiently removing nitrogen oxides from exhaust gases and other gaseous media.
Techniques used to remove nitrogen oxides from gaseous media typically involve selective catalytic reduction of the nitrogen oxides in the gaseous medium. Catalytic reduction of nitrogen oxides involves a multi-step process in which a reducing agent such as urea is first decomposed to ammonia and carbon dioxide. The ammonia then reacts with the nitrogen oxides to form molecular nitrogen and water, which is harmlessly emitted into the atmosphere.
One of the most effective methods used to remove nitrogen oxides from gaseous media is known as the Selective Catalytic Reduction (SCR) method. Various reducing agents which are applicable to the SCR method include ammonia, urea, hydrocarbon and the like. Catalysts which are suitable for the SCR method include metal oxide catalysts such as vanadia-titania type catalysts and various zeolite catalysts.
Recently, much research has been devoted to developing catalysts which have a high nitrogen oxide-removing capability across a broad temperature range. A need therefore exists for a catalyst which is capable of facilitating the efficient removal of nitrogen oxides from gaseous media at relatively low reaction temperatures, while remaining hydrothermally stable and catalytically active at relatively high reaction temperatures.